Inter-cell interference (ICI) from base station (BTSs) in cells adjacent to a serving cell is a performance-limiting impairment for users near cell edges in cellular networks, especially for smaller cells and a lower frequency re-use factor r (see, e.g., Khan, “LTE for 4G Mobile Broadband,” Cambridge University Press, 2009, and references therein). Simply put, frequency re-use calls for the overall bandwidth available to a cellular network to be divided into r frequency channels such that neighboring cells use different frequency channels (as possible) to minimize ICI. As r decreases, spectral utilization efficiency improves (since cells are configured to use more of the total bandwidth available to them) but ICI increases (since the number of neighboring cells using the same frequency channels increases). ICI adversely affects signal-to-interference ratio (SIR), reducing data rates and causing outages. When r reaches one, neighboring cells use the same frequency channels, and SIR and ICI tend to be at their worst.
To address this issue, ICI mitigation techniques have been developed to reduce outages (resulting in, e.g., dropped calls) and improve data rates (most popularly evidenced by higher Internet download speeds). Improved SIR also allows each BTS to cover more area while maintaining data rates. The expansion of each BTS's coverage region is especially attractive for rural cellular network deployments where the recently introduced OFDMA-based Long Term Evolution (LTE) cellular network standard supports a cell-size radius ranging from 5 km to 100 km, depending upon performance. In LTE vernacular, a BTS is known as an evolved-node base station (eNB).
In addition to ICI, the serving cell itself introduces interference to the user terminal by spatially multiplexing multiple data streams over the same time and frequency resources (see, e.g., Foschini, “Layered Space-Time Architecture for Wireless Communication in a Fading Environment When Using Multiple Antennas,” Bell Labs, Syst. Tech. J., vol. 1, p. 41-59, Autumn 1996), a technique called MIMO. These multiple data streams might be directed to a single user or divided between or among multiple users in the same cell. The former is called single-user MIMO (SU-MIMO), and the latter is called multi-user MIMO (MU-MIMO). In both scenarios, the user terminal encounters intra-cell interference in addition to ICI.
In the LTE and proposed LTE-Advanced (LTE-A) systems, r is set at one to provide users with higher spectral efficiency, but, again, at the cost of increased ICI. Like other cellular networks, intra-cell interference and ICI from eNBs in adjacent cells, especially smaller cells, is a performance-limiting impairment. To improve performance, LTE and proposed LTE-A systems allow eNBs to use multiple transmit antennas.
In its current incarnation, LTE supports three MIMO operating modes:
(1) a spatial beamforming mode (also known as a precoding mode) in which beamformer coefficients are designed to maximize the main lobe gain of a beamformer in the direction of the desired signal while introducing spatial nulls in the directions of the dominant interfering signals;
(2) a spatial multiplexing mode in which independent information streams are transmitted to maximize the data rate, but without the benefit of spatial diversity; and
(3) a spatial diversity mode in which a space-time block code (STBC) or space-frequency block code (SFBC) is used to transform information streams into correlated transmitted streams to enhance diversity in the presence of channel fading.
The spatial beamforming mode requires channel knowledge at the transmitter through a feedback channel and precoding. In contrast, the spatial multiplexing and spatial diversity modes do not require channel knowledge at the transmitter.
Unfortunately, the use of multiple transmit antennas at the eNBs introduces additional inter-antenna interference which should be carefully mitigated to achieve the best performance. This interference mitigation task is especially challenging for cell-edge users since they suffer from particularly significant ICI.